1. Field of the Invention
The present invention relates to a surface mount type package unit including a package body with a signal electrode directly connected to a wiring pattern formed on a circuit board through solder, and a method for mounting the package body on the circuit board.
2. Related Arts
In recent years, a lead-less type packaging structure utilizing solder bumps has been adopted to a surface mount type package unit for a semiconductor device and the like in place of a lead-type packaging structure utilizing leads (Gullwing leads, J-leads, and the like) to comply with a requirement for a high packaging density. In the lead-less type packaging structure, a package body usually has signal electrodes on the bottom face thereof to be directly connected to a wiring pattern on a circuit board. Accordingly, a packaging density on a circuit board is increased, and, for example, as in a ball grid array (BGA) package unit, it becomes possible to significantly increase a number of electrodes on the package body. In addition, design flexibility of the circuit board can be improved.
However, in such a packaging structure, a size of the package body itself has increased due to high density integration of semiconductor devices, necessity of mounting a transducer for sensing, and the like, resulting in increase of the self-weight of the package body. As a result, the following problem is caused. That is, the package body is bonded to a circuit board through solder portions (bumps). The solder portions are generally subject to thermal stress caused by a difference in thermal expansion coefficient between the package body and the circuit board to have thermal fatigue failure therein. Further, it is generally known that increasing the thickness of the solder portions (height of the bumps) is effective to prevent the thermal fatigue failure of the solder portions.
However, in the case where the self-weight of the package body is large, the thickness of the solder portions is decreased during a fused process for mounting the package body on the circuit board. As a result, the thermal fatigue failure of the solder portions are likely to occur, resulting in deterioration of reliability at a mounted state. This problem makes the surface mount type package unit difficult to be adopted to a device such as an electric control unit for a vehicle, because such a device is used in hard environmental conditions including large change in temperature, large amount of vibrations, and the like.
On the other hand, in the lead-less surface mount type package unit, there is another problem such that it is difficult to check the bonding state of the package unit by solder from the appearance thereof. In the case where the bonding state of the package unit is checked only by an electric characteristic test, it is difficult to prevent increase of percent defective of the package unit at the mounted state. As a result, the reliability of the mounting performance of the package unit deteriorates.